1. Field of the Invention
The present invention relates to a magnetic random access memory (MRAM) using the magnetoresistive effect, and a method of manufacturing the same.
2. Description of the Related Art
Recently, many types of memory for storing information by the application of new principles have been proposed. One such type, as proposed by Roy Scheuerlein et al., is the magnetic random access memory using the tunneling magnetoresistive (TMR) effect. (See non-patent reference 1.)
A magnetic random access memory stores a binary value in a magnetic tunnel junction (MTJ) element. The MTJ element has a structure in which two magnetic layers (ferromagnetic layers) sandwich an insulating layer (tunnel barrier). Whether value stored in the MTJ element is binary 1 or 0 is determined in accordance with whether the spin directions in the two magnetic layers are parallel or antiparallel.
Data written in the MTJ element is read as follows. Switching elements are connected in series with MTJ elements, and only a switching element connected to a selected read word line is turned on to form a current path. Since a current flows to only the selected MTJ element, data can be read from the MTJ element.
When a MOSFET is used as the switching element, the cell size is 12F2 if the short side (the width in the magnetic hard axis direction) of the MTJ element is F (Feature size) and the long side (the width in the magnetic easy axis direction) is 2F. Accordingly, the cell size of the magnetic random access memory is larger than those of a DRAM and flash memory. The cell size becomes 10F2, therefore, if an easy axis write bit line is formed below the MTJ element and the lower electrode of the MTJ element and the fringe of the contact of the lower electrode are self-aligned. However, the decrease in cell size is still unsatisfactory.
[Non-patent reference 1] Roy Scheuerlein et al., ISSCC2000 Technical Digest p. 128, “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”
[Patent reference 1] Jpn. Pat. Appln. KOKAI Publication No. 2005-175357